1. Field of the Invention
The hermetic sealing cover of the invention is of particular utility in hermetically sealing a container in which is enclosed one or more semiconductor devices, such as integrated circuits which must be protected from exposure to ambient atmosphere.
2. Description of the Prior Art
In the manufacture of semiconductor devices, there has been a requirement for hermetically sealing the container in which the active semiconductor device is housed, usually a cavity in a metallic or ceramic body. For this purpose, a metallic cover and a solder preform ring have been used. In accordance with current general practice, the solder preform ring is preattached to the sealing cover and the resulting unit disposed over the container of the semiconductor device and heated to fuse the solder preform to the cover and to the container as described and claimed in applicant's U.S. Pat. Nos. 3,823,468 and 3,874,549.
In the foregoing method of fabricating hermetically sealed semiconductor packages, it has been customary to use a solder preform formed of an eutectic alloy comprising substantially 80% gold and 20% tin, which has the advantages that it has a desirably high melting point of about 280.degree. C. and it has a high tensile strength, ensuring against breaking of the seal due to shocks or rough handling of the semiconductor package. While such solder preforms are eminently satisfactory in use, the recent extreme increase in the price of gold has made them very costly.
One modification of the foregoing practice adopted to effect a cost saving and which has gone into commercial use comprises a nickel-plated metal cover having an attached preform of substantially chemically unreacted plastic material such as an epoxy resin, as described and claimed in applicant's U.S. Pat. No. 4,109,818 issued Aug. 29, 1978. An outstanding advantage of such a plastic preform is the extremely good adhesion that obtains between the cover and the cavity sealing ring of the semiconductor package. However, from a rigorous viewpoint, the seal effected by such a plastic preform cannot be considered truly hermetic. For example, a gold-tin eutectic solder preform produces an hermetic seal such that the leak rate does not exceed 1.times.10.sup.-8 c.c. of helium per second at a differential pressure across the seal of the order of one atmosphere, while the plastic sealing rings are customarily only warranted to a leak rate of 1.times.10.sup.-6 c.c. of helium per second. In some applications, this higher leak rate is acceptable while in other applications the leak rate of 1.times.10.sup.-8 c.c. of helium per second is required.
It is an object of the present invention, therefore, to provide a new and improved hermetic sealing cover for a container for a semiconductor device which combines the superior adhesion properties of a plastic sealing ring with the superior hermetic sealing properties of a metallic solder preform.